Electrodialysis Stack Research Articles

Numerical Investigation of the Effect of Two-Dimensional Surface Waviness on the Current Density of Ion-Selective Membranes for Electrodialysis

Ion-selective membranes are an important component of electrodialysis stacks for desalination. Manufacturing imperfections or slight inhomogeneity of the material can lead to minute membrane surface imperfections. Two-dimensional solutions of the coupled Poisson–Nernst–Planck and Navier–Stokes equations were sought for a perfectly smooth membrane and for membranes with well-defined small-amplitude harmonic surface roughness. The simulations were carried out with the validated rheoEFoam solver by Pimenta and Alves. In the overlimiting regime, the electric field is strong enough for an electrokinetic instability to occur. The instability leads to disturbance growth and the formation of electro-convection cells, which strongly increase the current density. The present simulations show that with an increasing ion concentration and applied voltage, the instability becomes stronger and the overlimiting regime is reached earlier. The limiting current density shows a noticeable dependence on the wavelength of the surface roughness. When the wavelength of the surface roughness is incommensurate with the wavelength of the naturally occurring instability, the limiting current density is increased. Since production membranes will always have some degree of surface roughness, this suggests that membrane surface treatments which favor certain wavelengths may have an effect on the overall membrane performance.

Application on-line NIR spectroscopy and other process analytical technology tools to the characterization of soy sauce desalting by electrodialysis

Application of Quality by Design (QbD) approaches to food processes requires the development and identification of complex mathematical models. With reference to soy sauce desalting by electrodialysis (ED), the aim of this work was to combine several Process Analytical Tecnology (PAT) tools, such as Near Infrared spectroscopy (NIR), level and conductivity probes, and a control strategy of the electric current generator to increase the amount of information of single desalting experiments for ED modelling. Soy sauce volume and electric conductivity as well as the electric voltage and current applied to the ED stack were precisely measured at high sampling rates. NIR models of the main soy sauce physicochemical properties (density, osmotic pressure, electric conductivity; salt and non-salt solid concentrations), needed for ED modelling, were developed and applied to several experiments resulting in good predictions. The results indicate that the demonstrated procedure enhances ED process characterization for desalting of soy sauce or other food systems.

Fractionating various nutrient ions for resource recovery from swine wastewater using simultaneous anionic and cationic selective-electrodialysis

Different from current nutrient recovery technologies of recovering one or two nutrient components (PO43− or NH4+) from wastewater, this study aimed to fractionate various nutrient anions and cations simultaneously, including PO43−, SO42−, NH4+, K+, Mg2+ and Ca2+, into several streams. The recovered streams could be further paired together to produce high-value products. A novel electrodialysis process was developed by integrating monovalent selective anion and cation exchange membranes into an electrodialysis stack. Results revealed that nutrient recovery was achieved effectively by fractionating PO43− and SO42− into the anionic product stream, whereas bivalent cations (Mg2+ and Ca2+) were extracted in the cationic product stream and the monovalent cations (K+ and NH4+) were concentrated in the brine stream. For the permeation capabilities of anions, SO42− and Cl− possessed the higher preference, whereas PO43− permeated the membrane more difficult. As to the cations, the permeation sequence was: NH4+≈K+ >Ca2+>Mg2+≈Na+. Enhancing voltage values not only promoted ion migration rates, but also led to the increase of energy consumption. Although elevating initial phosphate concentration in the anionic product streams from 60 mg/L to 470 mg/L did not influence phosphate fractionation significantly, the current efficiency decreased from 3.55% to 0.65% and a remarkable increased of energy consumption from 29.42 kWh/kg NaH2PO4 to 160.13 kWh/kg NaH2PO4 was observed. Further experiments were conducted for phosphorus recovery by pairing two recovered product streams, which revealed that phosphate precipitation could be achieved by using inherent Ca2+ and Mg2+ in the wastewater without dosing external cation sources.

Green fabrication of pore-filling anion exchange membranes using R2R processing

Pore-filling ion exchange membranes are fabricated by complicated and energy-inefficient processes, use of large amounts of polar organic solvents, and optionally post-modification using acid or base. For green fabrication of pore-filling anion exchange membranes (PAEMs), we optimized the process parameters such as pretreatment time, impregnation time, water amount, and photo-polymerization rate of each step using roll to roll (R2R) equipment. Based on these optimized process parameters, a PAEM of 57.5 cm width and 39 μm thickness completely filled with a photo-cured electrolyte polymer was fabricated by aqueous pretreatment and by using an impregnation solution without toxic organic solvents. The impregnation step was conducted along with photo-polymerization at a line speed of 0.3 m/min without repeated impregnation with R2R equipment. The IEC, resistance, and permselectivity of this PAEM were similar to those of handmade PAEMs. The PAEM exhibited chemical stability in the pH range of 0–12. In addition, the reverse electrodialysis stack assembled with these PAEMs exhibited a higher power density than a stack of commercial ion exchange membranes. These results demonstrate that industrial-scale PAEM can be fabricated through a rapid, simple, environmentally friendly, and energy-efficient R2R process.

Electrodialysis-based zero liquid discharge in industrial wastewater treatment.

Over the past few decades, reverse osmosis (RO) has been the dominant technology employed in zero liquid discharge (ZLD) systems for industrial wastewater treatment (WWT). However, RO is limited to a maximum operating salinity of about 75 g kg-1. Electrodialysis (ED) is a potentially attractive option as it can achieve much higher concentrations, thereby reducing the capacity and energy demand of the subsequent evaporation step. Feed-and-bleed experiments were undertaken on a laboratory-scale ED stack using a series of model solutions based on the most common inorganic salts with the aim of determining maximum achievable concentrations. The maximum salt concentration achievable via ED ranged between 104.2 and 267.6 g kg-1, with levels predominantly limited by water transport. In addition, a straightforward review of how ED incorporation can affect ZLD process economics is presented. The operational cost of an ED-based ZLD system for processing RO retentate was almost 20% lower than comparable processes employing high-efficiency RO and disc tubular RO. As the ED-based ZLD system appears economically preferable, and as maximum achievable concentrations greatly exceeded RO operating limits, it would appear to be a promising approach for bridging the gap between RO and evaporation, and may even eliminate the evaporation step altogether.

Cationic selectrodialysis for magnesium recovery from seawater on lab and pilot scale

In view of its multiple applications the selective recovery of Mg2+ from seawater can be very interesting. For this purpose, cationic selectrodialysis (cSED), which uses a conventional electrodialysis stack that is supplementary equipped with monovalent selective cation-exchange membranes, was used. Initially, various parameters were investigated on lab scale using synthetic seawater. The results pointed out that a lower current density resulted in a higher Mg2+ concentration in the product. A higher unwanted Mg2+ concentration was obtained in the brine when the initial concentration of NaCl in the product was lowered. Subsequently, it was demonstrated that the treatment of real North Sea water with cSED was technologically feasible during a short-term batch experiment on lab scale. In a next phase, a cSED pilot plant was operated in a stable way in a long-term feed and bleed experiment in which the Mg2+ content of the North Sea water that was used as initial product was at least doubled. However, the Ca2+ concentration in the product also increased. This might reduce the applicability of the recovered product. From this research it is concluded that cSED is a promising and attractive technology to recover Mg2+ from seawater.

The feasibility and mechanism of reverse electrodialysis enhanced photocatalytic fuel cell-Fenton system on advanced treatment of coal gasification wastewater

A novel reverse electrodialysis enhanced photocatalytic fuel cell (PREC)-Fenton system is established for advanced treatment of coal gasification wastewater. Results indicate the reverse electrodialysis (RED) stack is crucial to achieve higher electricity production and H2O2 generation. Operational parameters including salinity gradient, air flow rate, solutions flow rate and pH are investigated. The optimal H2O2 production and wastewater treatment are observed at salinity ratio of 100, air flow rate of 16 mL/min, solutions flow rate of 2.0 mL/min and a broaden pH of 2–7. A high coulombic efficiency of 59.6% and cathodic efficiency of 51% is obtained, while the COD, total phenol and NH4+-N removal reach 92.4%, 86.1% and 77.3%. Moverover, the system requires much lower energy consumption of 26.4 kWh/(kg·TOC). The PREC-Fenton system is a more economical and efficient candidate for energy recovery and wastewater treatment.

Performance investigation and optimisation of electrodialysis regeneration for LiCl liquid desiccant cooling systems

In this paper, the regeneration performance of membrane electrodialysis (ED) with three different types of ion-exchange membrane pairs working with LiCl liquid desiccant solutions was first experimentally investigated. The membrane pair with the optimal performance for concentrating desiccant solutions was then identified and selected to quantitatively examine the effect of the operating parameters of ED on salt and water mass transfer, the concentration of the transferred solution and energy consumption of the ED stack using Taguchi method and analysis of variance. An optimisation was also performed to determine the optimal levels of the operating factors. The results showed the supplied current density significantly influenced the energy consumption of the ED stack but had a limited impact on the concentration of the transferred solution. Under the predicted optimal operating conditions, the energy consumption can be saved by 31.7% while the concentration of the transferred solution increased by 9.4% in comparison to the average values in the studied range. The main findings can be potentially used to guide optimal design and operation of ED regeneration for LiCl liquid desiccant cooling systems.

On-line construction of electrodialyzer with mono-cation permselectivity and its application in reclamation of high salinity wastewater

Electrodialysis (ED) technology has been paid special attention in reclamation of high salinity effluents. Therein, the attempts on how to separate out scale-forming cations are never ending. Different from a conventional scheme, an on-line endowment of mono-cation permselectivity to a conventional ED stack was put forward in this work by electro-deposition of branched polyethyleneimine with an optimized molecular weight. Subsequently, series of work performances, including limiting current, mono-cation permselectivity, permselectivity between specific anions, water transport behavior, current efficiency and energy consumption, were investigated in reclamation of a typical saline wastewater. The pilot-scale ED experiments showed that the modified ED stack displayed a significant mono-cation permselectivity, for examples, PNa+Ca2+ and PNa+Mg2+ were reduced from 0.36 and 0.81 to 0.11 and 0.12, respectively. Interestingly, a certain permeability priority for SO42- was also achieved at the same time. Moreover, the five-month service life tests indicated the ED stack can maintain a stable separation performance. On the other hand, some problems induced by the modification were also revealed, such as the facilitated water transport and the increase of energy consumption. Anyway, it is reasonable to believe that the scheme should have broad and promising application perspectives due to its effectiveness, economy and simplicity.

Fabrication of photocured anion-exchange membranes using water-soluble siloxane resins as cross-linking agents and their application in reverse electrodialysis

The utilization of large amounts of volatile organic solvents and the complicated process required for industrial manufacturing of ion-exchange membranes necessitate the development of simple, rapid, and environmentally friendly fabrication methods such as those based on photopolymerization. We employed hydrolytic sol–gel reactions between ammonium- and acrylamide-functionalized silane coupling agents to synthesize water-soluble siloxane resins that exhibit high condensation levels (>80%) and comprise oligomers with molecular weights below 2000 Da. These resins were then mixed with a hydrophilic monomer bearing ammonium and acrylamide groups, and porous polyethylene substrates were impregnated with the resulting mixtures and then irradiated with ultraviolet light. The hydrophilicity, mechanical strength, and other properties of the resulting membranes depended on the resin composition, indicating that the substrate pores were efficiently filled with the prepared resins and further suggesting that the membrane performance could be effectively altered by varying the resin composition. Moreover, the obtained membranes exhibited chemical stability in the pH range between 0 and 11 and in hot water at 60 °C. The reverse electrodialysis stack consisting of these membranes showed higher power density than a stack of commercial membranes. Therefore, it can be concluded that without employing volatile organic solvents for reverse electrodialysis, the developed technique is well-suited for the fabrication of ion-exchange membranes.

Layer-by-layer coatings on ion exchange membranes: Effect of multilayer charge and hydration on monovalent ion selectivities

Electrodialysis (ED) is an important process to desalinate brackish water, which contains scale-forming divalent ions. Hence, there is an interest in separating mono- and divalent ions. In recent years, layer-by-layer (LbL) polyelectrolyte coatings on ion exchange membranes (IEMs) have provided high monovalent ion selectivities. However, there is a lack of understanding on the structure-property relationships for polyelectrolyte multilayers for membrane applications and lack of evaluation under practical conditions. In this work, we evaluate polyelectrolyte multilayer properties by optical techniques, connect them to monovalent-selectivity using resistance measurements and upscale this to practical monovalent-selectivities for an ED unit. Excess positive poly(allyl amine) (PAH) was observed in the PAH/PSS (polystyrene sulfonate) multilayer, resulting in a positively charged multilayer. Coated multilayers with low hydrations (0.2) are able to achieve high monovalent-selectivities (of 5.7 up to 7.8) on CEMs comparable to the commercial monovalent-selective CSO (with 6.9). This enhanced selectivity was only observed for cations with LbL-coated CEMs and not for anions with LbL-coated AEMs, which we argue is due to the excess positive charge of the PAH/PSS multilayers. Finally, LbL-coatings show an improved monovalent-selectivity in ED stack experiments with artificial brackish water of 1.7 for LbL-coated CEMs compared to uncoated CEMs (with a monovalent-selectivity of 0.5).

Reverse Electrodialysis Energy Harvesting System Using High-Gain Step-Up DC/DC Converter

Salinity gradient power (SGP) between fresh river water and sea water is a form of renewable energy with huge potential but not well explored. This paper presents a feasibility study on energy harvesting of SGP based on: 1) the use of reverse electrodialysis (RED) stack, and 2) the combined use of harmonics-boosted resonant inverter and multistage diode-capacitor step-up converter. The properties of an RED stack have been characterized into steady-state ac and dc equivalent circuit models for power converter design for the first time. The gains of the resonant inverter and diode-capacitor step-up converter are also optimized for maximizing the energy efficiency. An RED stack prototype comprising multiple alternating anion and cation exchange membranes with an area of 0.01 m $^{\text 2}$ each has been constructed. The dc output voltage of 2–3 V from the RED stack has been stepped up to be over 155 V. This study has confirmed that energy can be harvested with a membrane power density of at least 1.4 W/m2, a power converter's efficiency exceeding 85%, and a voltage gain of 67.3 times.

Effect of ions (K+, Mg2+, Ca2+ and SO42−) and temperature on energy generation performance of reverse electrodialysis stack

For investigating the influence of coexisting ions (K+, Mg2+, Ca2+ and SO42−), temperature and their synergistic effect on energy generation by reverse electrodialysis, two series of cells dividing with Yadeshi membranes are fed with different solutions at 10 °C, 25 °C and 40 °C. The presence of K+, Mg2+, Ca2+ and SO42− results in lower open circuit voltage, higher internal resistance and lower maximum power density, and the influence order of ions coexisting with NaCl is Ca2+ > Mg2+ (>SO42−) > K+. With the temperature risen, the open circuit voltage and the internal resistance show a trend of increase and decrease respectively, resulting in a bigger power density. Based on the synergistic effect of coexisting ions and temperature, the maximum power density of the pure NaCl system shows a greater increment (0.15 W m−2) than that of NaCl-CaCl2 (0.10 W m−2) and NaCl-MgCl2 (0.11 W m−2) systems when temperature increases from 10 °C to 40 °C. Furthermore, the transport quantities of ions in each system increased with temperature at different degrees, and the uphill of Ca2+ and Mg2+ was more obvious, which can reasonably explain the different effects of temperature on the maximum power density. Moreover, these results are further verified when simulated concentrated seawater is used for both the Yadeshi- and Fujifilm membranes, and the Fujifilm shows better energy generation performance mainly due to a lower internal resistance.

Fractionating magnesium ion from seawater for struvite recovery using electrodialysis with monovalent selective membranes

As the consumption of global phosphorus reserves accelerates, recovering phosphorus as struvite (MgNH4PO4·6H2O) from wastewater is an important option for phosphorus recycling. However, magnesium source is one of the major limiting factors for struvite recovery. In this work, different from previous studies where seawater was used directly as magnesium source in struvite precipitation, an electrodialysis stack equipped with monovalent selective cation-exchange membranes was designed to fractionate Mg2+ from seawater for struvite recovery. Results revealed that Mg2+ fractionation was achieved effectively. The comparison on applying the driving force for ionic transport showed that constant voltage was more preferable than constant current due to its higher Mg2+ separation efficiency, current efficiency and lower energy consumption. Increasing voltage from 7 V to 13 V would improve Mg2+ permeation ratio from 72.9% to 80.5% into the product stream but simultaneously increased the energy consumption from 5.40 (kWh/kg MgCl2) to 11.69 (kWh/kg MgCl2). In addition, the investigation on the influence of Ca2+ co-existence and further struvite recovery experiments revealed that the variation of Ca2+ concentrations in seawater did not influence Mg2+ fractionation significantly, nevertheless it might reduce struvite recovery efficiency through forming calcium phosphate.

Performance analysis of reverse electrodialysis stacks: Channel geometry and flow rate optimization

In this paper, the optimal channel geometry and flow rate of the concentrated and diluted solutions under the maximum net power output for the reverse electrodialysis (RED) stacks at a confined size are systematically investigated. A model considering the change in volume flow rate along the flow direction is employed to illustrate the process of the RED stack. For systematisms, under the maximum net power output, we first consider the optimal channel thicknesses at a given identical inlet flow rate and then the optimal channel thicknesses and flow rates. The profiles of flow rate, concentration, power density, and hydrodynamic loss along the flow direction are discussed. The net power output and energy efficiency for different membranes under the above two optimization situations are analyzed and compared. The results reveal that the optimal channel thickness of the high-concentration (HC) compartment is slightly larger than that of the low-concentration (LC) compartment for a given identical inlet flow rate. When both channel thickness and flow rate are optimized, the optimal channel thicknesses and volume flow rates of the HC compartment are, respectively, less than those of the LC compartment and the net power output and energy efficiency are significantly improved.

Influence of temperature gradients on mono- and divalent ion transport in electrodialysis at limiting currents

Temperature gradients in electrodialysis (ED) stacks can potentially enhance the efficiency of charge separation and the selective transport of ions. We have previously investigated temperature gradients in the Ohmic regime but not in the limiting current regime, where diffusion of ions towards the membrane determines the transport rate and temperature gradients potentially have the largest influence. In this research, commercial ion exchange membranes (FAS and FKS, FUMATECH, Germany) are used for the investigation of temperature gradients in the limiting current regime. In contrast to the Ohmic regime, we find that heating the diluted stream increases the current obtained (at a constant applied potential) when compared to heating the concentrate stream in systems containing monovalent KCl and NaCl solutions. For mixtures of mono- and divalent ions, the temperature gradient has a larger influence on the selectivity of the separation. If the desalinated stream is heated, divalent Mg2+ ions show a higher transport than the monovalent K+ and Na+ ions. This is due to the enhanced competitive transport of the mono- and divalent ions under the application of a temperature gradient. These results show the potential application and relevance of temperature gradients to enhance the selective separation of mono- and divalent ions.

A robust model of brackish water electrodialysis desalination with experimental comparison at different size scales

This paper presents a robust analytical model for brackish water desalination using electrodialysis (ED), with prediction of the desalination rate, limiting current density, and total energy use including pumping energy. Several assumptions reduce computation time and accurately model ED system behavior. The predicted desalination rate, limiting current density, and total energy usage agree with measurements across two diverse ED stack designs, differing in total membrane area (0.18 m2, 37.1 m2), membrane manufacturers (GE Water, PCA GmbH), and flow channel spacers. The commercial-scale stack was additionally tested with real groundwater, demonstrating that brackish groundwater may be modeled as an equivalent concentration NaCl solution. Sensitivity to the membrane diffusion coefficient, area available for ion transport, level of discretization along the flow channel length, boundary layer and membrane resistances, and water transport are analyzed to guide empirical characterization when higher accuracy is required. No single existing model for pressure drop in the membrane spacers could accurately predict pumping power in both stacks. One model for each stack was found to reasonably approximate pressure drop, however experimental validation of specific spacer designs is recommended. The fully quantitative, parametric description of electrodialysis behavior presented forms a useful tool to design, evaluate, and optimize ED systems.

Membrane Separation of Ammonium Bisulfate from Ammonium Sulfate in Aqueous Solutions for CO2 Mineralisation

The separation of ammonium bisulfate (ABS) from ammonium sulfate (AS) in aqueous solutions by monovalent ion selective membranes was studied. Optimised usage of these chemicals is both an important and challenging step towards a more efficient CO2 mineralisation process route developed at Åbo Akademi University (ÅA). The membranes were placed in a three or five-compartment electrodialysis stack. Silver, stainless steel and platinum electrodes were tested, of which a combination of Pt (anode) and stainless steel (cathode) electrodes were found to be most suitable. Separation efficiencies close to 100% were reached based on ABS concentrations in the feed solution. The tests were performed with an initial voltage of either 10 V–20 V, but limitations in the electrical power supply equipment eventually resulted in a voltage drop as separation proceeded. Exergy calculations for energy efficiency assessment show that the input exergy (electrical power) is many times higher than the reversible mixing exergy, which indicates that design modifications must be made. Further work will focus on the possibilities to make the separation even more efficient and to develop the analysis methods, besides the use of another anode material.

Application of electro-membrane separation for recovery of acetic acid in lignocellulosic bioethanol production

Acetic acid (HAc), one of the major inhibitory compounds present in all lignocellulosic biomass hydrolysate, can reduce the rate and yield of bioethanol production. Electrodialysis (ED) is an electrochemical non-solvent based separation process that can selectively separate ionic species from aqueous solutions through the use of ion selective membranes and electric field potential. Two ED cell configurations at three constant applied potentials (5, 10, and 15V) were tested for removal and recovery of HAc from a model solution of corn stover hydrolysate. The uses of bipolar membrane (configuration 1) and cation-exchange membrane (configuration 2) in an ED stack demonstrated significantly different effects on system performance in terms of: demineralization rate, acetic acid removal rate, current efficiency, and energy consumption. The demineralization and mineralization of feed and recovery solutions, respectively, increased with increasing applied voltage for both configurations. Configuration 1 showed 1.5–2 times higher initial electrical resistance at lower applied voltages compared to configuration 2. The HAc flux and removal rate increased with increasing electric potential, and were significantly higher in configurations 2 than 1. The present work indicates that a cation exchange membrane operating at 10V is optimal for recovering HAc from a model corn stover hydrolysate solution.

Effects of hypochlorite exposure on the structure and electrochemical performance of ion exchange membranes in reverse electrodialysis

We performed chlorination experiments to understand the impact of hypochlorite exposure on both cation exchange membranes (CEMs) and anion exchange membranes (AEMs) regarding their properties and performance in a reverse electrodialysis (RED) stack. Changes in membrane morphology, surface elemental composition and chemical bounding suggested the chlorine incorporation in the form of C-Cl bonds and side-chain cleavage of –SO3- or –NR3+ containing molecular fractions. These observations were further supported by observed increases in hydrophobicity and decreases in fixed charged groups of the membranes, respectively. Compared to CEMs, AEMs were less chlorine resistant such that the development of more extensive cracks in the membrane structure further increased water content and dramatically decreased membrane conductivity. The performance of both chlorinated CEMs and AEMs were tested in an RED stack and the results showed the reduced RED power density was largely attributed to the deteriorated electrical properties of AEMs.